In the railway car field, there is increasing use of a car control unit (CCU) at each car in a train of cars for controlling the braking of cars. The CCU is electrically connected to a Head End Unit (HEU) in the locomotive for control and indicating purposes. See, for example, U.S. Pat. Nos. 5,335,974 and 5,503,467.
Normally, the brakes are applied by an air operated piston and cylinder assembly which is connected to the brakes by a series of levers and beams. Brake rigging ratios to increase braking force, along with the 5/8" allowable wear of eight shoes, will produce major rigging component displacement problems and excessive piston stroke. It is, therefore, necessary to include a brake slack adjuster in the mechanism or "rigging" which interconnects the piston and cylinder assembly with the brake shoes. See, for example, U.S. Pat. Nos. 3,850,269; 4,312,428 and 3,283,861. Normally, the pressure of the air in the piston and cylinder assembly is an indication of whether the brakes are applied or released. However, such air pressure, e.g. when it is relatively low or zero, does not necessarily indicate that the brake shoes have become disengaged from the car wheels because there may be a fault in the brake shoe driving mechanism which, even though the air pressure in the piston and cylinder assembly is low or zero, will not permit the brake shoes to move away from the wheels. In the event of such a fault, the brake shoes and the wheels which they engage will overheat when the car is put in motion. Accordingly, an indication at the Head End Unit (HEU) that the bar brakes have been released, which is based on the air pressure in the piston and cylinder assembly, is not always reliable.
When the brake rigging includes a brake slack adjuster, it is disposed intermediate the brake force generator, i.e. the piston and cylinder assembly, and the brake beams which support and move the brake shoes. The trigger or actuator return spring of a brake slack adjuster usually has a size and strength sufficient to return the levers, etc., of the brake rigging to the full release position provided that there are no extraneous forces which prevent full release. In other words, the position of the actuator or trigger is a direct, rather than an indirect, indication of brake shoe disengagement from the car wheels.